This invention relates generally to radio receiving apparatus for processing spread spectrum signals contaminated by narrowband interference, and in particular, to direct sequence/spread spectrum systems in which each data bit is mapped into a pseudorandom noise sequence of binary pulses (chips).
Spread spectrum signals are used in digital radio systems for telecommunication and navigation purposes. In navigation systems (Global Positioning System (GPS), GLONASS, GPS/GLONASS), a receiver processes several spread spectrum signals, each one emitted by a different satellite, to track the distance of the receiver from each satellite, and thereby, to determine its own position. In telecommunication systems, spread spectrum signals are used for (i) combating interference, (ii) transmitting at very low power to avoid detection/interception, and (iii) multiplexing one channel over many users.
Spread spectrum signal processing is characterized by expanding the bandwidth of the transmitted signal by a large factor (typically higher than 100) through pseudorandom noise (PN) modulation, and by compressing the bandwidth of the received signal by the same factor. PN modulation is implemented by two techniques: (i) by transmitting a PN sequence of binary pulses in each data bit interval, which is referred to as direct sequence/spread spectrum (DS/SS) system, and (ii) by employing different carrier frequencies in each data bit interval (or fraction thereof) so that the record of used carrier frequencies constitutes a PN sequence, which is referred to as a frequency hopped/spread spectrum (FH/SS) system. In both methods bandwidth compression is accomplished by correlating the received signal with the known PN chip or carrier frequency sequence. Due to spread spectrum signal processing, every incident interference is spread at the receiver over the whole system bandwidth, whereas the bandwidth of the desired signal is compressed. As a result, the effective interference power is smaller than the total incident interference power by a factor equal to the bandwidth expansion factor. For the above reason, the bandwidth expansion factor is referred to as the processing gain of the spread spectrum system.
In a DS/SS receiver it is possible to suppress a narrowband interferer beyond the processing gain, by filtering the received signal prior to despreading through an adaptive transversal filter (ATF). An ATF estimates the interference component in a reference input sample X.sub.i through an optimal linear combination X.sub.i =(W.sub.N X.sub.i-N +. . . +W.sub.1 X.sub.i-1) +(W.sub.-1 X.sub.i+1 +. . . +W.sub.-N X.sub.i+N) of N delayed input samples (X.sub.i-1, . . . , X.sub.i-N) and N advanced input samples (X.sub.i+1, . . . , X.sub.i+N), which are typically spaced one-chip interval apart. Interference is suppressed by subtracting the estimate X.sub.i from the reference X.sub.i and the difference Y.sub.i is the ATF output, i.e., Y.sub.i =X.sub.i -X.sub.i. If the interference is estimated from delayed input samples only (i.e., as in prediction filtering), the ATF is referred to as a one-sided ATF. If advanced input samples, as well as delayed input samples, are used (i.e., as in interpolation filtering) then the ATF is referred to as a two-sided ATF.
Besides suppressing interference, the above filtering increases the thermal noise and distorts the PN-code in ATF output Y. The ATF gain corresponds to the net benefit of interference suppression minus the signal-to-noise ratio (SNR) losses due to excess thermal noise and PN-code distortion. The ATF gain increases as the PN-code components of the combined samples become less correlated and the interference components become more correlated. Since the correlation between consecutive signal samples increases as the signal power spectrum gets narrower, significant ATF gain is expected when the interference spectrum occupies a small fraction, typically less than 10%, of the PN-code bandwidth. In the frequency domain, the operation of ATF corresponds to discriminating against the interference spectrum by forming a linear filter (through the appropriate weights W.sub.k) with a notch around the center frequency of the interferer. Accordingly, ATF is not very effective against wideband interference, but it is very effective against continuous-wave (CW) interference and other narrowband interferences, such as pulsed CW, swept CW, and narrowband noise (AM or FM).
Assuming that the interference has an adequately narrowband spectrum for ATF application, there are two critical system requirements for achieving significant ATF gain. First, there must be adequate means for filtering and, secondly, there must be adequate means for generating automatically appropriate weights. Regarding the filtering aspect thereof, the ATF must be capable of combining a minimum number of input samples to estimate interference. If the interference forms K well-separated spectral bands, the minimum number of taps is 2K because the ATF needs to introduce at least one spectral notch at each interference band, at the expense of two taps (i.e., real weight coefficients) per notch. Regarding automatic weight generation, the optimal weights depend on the interference characteristics and on the criterion of optimality or cost function. An effective cost function for spread spectrum systems subjected to strong interference is the average power of the ATF output signal Y. Classical mean square error theory shows that the optimal weights can be obtained by solving a set of 2N linear equations (normal equations), which involve the correlation function of the ATF input signal. A practical iterative algorithm, which has been shown to converge (on the average) to the optimal weights, is the Widrow-Hoff algorithm. This algorithm updates W.sub.k as: W.sub.k =OLD(W.sub.k) +u.cndot.X.sub.i-k .cndot.Y.sub.i. The parameter (u) is referred to as the step-size (of the algorithm), and it controls the convergence characteristics and the steady-state weight jitter of the algorithm. It has been determined, through ATF simulations in multiple CW interference, that the typical value of u=0.01 is a good compromise between convergence rate and steady-state jitter.
Although the theoretical principles of adaptive transversal filtering were introduced almost 30 years ago, it is desirable to develop better filtering techniques so as to reduce the cost and improve the performance thereof, as well as to expand the use thereof into many new applications. In a previous invention (5,268,927, December 1993, Dimos, et. al.), the applicants proposed a digital ATF which could be incorporated in a spread spectrum receiver, and which could achieve a high level of suppression of narrowband interference by proper design of signal regulation, signal resolution, and prevention of ATF weight drifting. Although integration of a digital ATF in a spread spectrum receiver is the most efficient approach for interference suppression for new receiver designs, this is not necessarily true for existing receiver designs. In particular, regarding safety-critical equipment (for example, GPS receivers for aircraft navigation), the cost of redesigning and recertifying that equipment is very high. It is therefore very desirable to perform interference suppression through the digital ATF without having to modify an existing receiver.